The average structure of ternary sul5de Cu 2 Gd 2/3 S 2 has been re5ned using single-crystal X-ray di4raction data: space group P3 , a β«Ψβ¬ b β«Ψβ¬ 3.899(1), c β«Ψβ¬ 6.421(2) A > , Z β«Ψβ¬ 1, R F β«Ψβ¬ 0.036, and wR F β«Ψβ¬ 0.041 for 418 re6ections and 8 structural parameters. The average structure is based on hexagonal close packing of S with Cu in tetrahedral sites and Gd in octahedral sites, the latter sites being occupied to 2/3. Di4use streaks observed by electron diffraction, however, indicate a trigonal strict periodicity of A β«Ψβ¬ 6.753 A > β«Ψβ¬ (3a within (001) and a vague period of C β«Ψβ¬ 12.842 A > β«Ψβ¬ 2c, suggesting a layer structure with an intralayer order and interlayer short-range order of Gd vacancies. A model of the real structure has been proposed, and the di4use scattering has been simulated by the matrix method for onedimensional disorder. The calculated intensity distribution corresponds to the observed distribution of the di4use streaks in the electron di4raction pattern and to the intensity distribution measured on an X-ray single-crystal di4ractometer; the total X-ray powder di4raction pattern of Cu 2 Gd 2/3 S 2 can be reproduced almost completely by calculation.